Abstract
This study examines the influence of composite action between steel beams and cross-laminated timber (CLT) slab on progressive collapse behaviour. A six-storey steel frame, originally designed as a bare steel frame (BSF) for steel-concrete composite floors, was first validated and then redesigned as a steel-timber composite (STC) system. Floors were modelled using CLT shell elements in both configurations. In the STC model, composite action was enabled through nonlinear spring connectors between CLT slab and steel beams, calibrated using experimentally validated load-slip relationships for coach screws. Progressive collapse performance was evaluated through nonlinear static push-down assessments under edge, corner, and internal column removal scenarios. The STC frame with 30% composite action exhibited up to 48% lower displacements and up to 14% higher vertical load resistance than the BSF for internal and edge column removals. For corner column removals, ultimate capacities were similar, but the STC frame achieved up to 44% lower displacements. Sensitivity studies were carried out in which the composite action was varied between 15–70%. Increasing the composite action from BSF to 15% and 30%, reduced displacements by 21% and 38% on average, respectively, and increased vertical load resistance by up to 14%, while 60–70% composite action mainly improved deformation capacity. The results indicate that moderate levels of composite action provide the greatest benefit to robustness.